1. Field of the Invention
The present invention relates to a printing device, such as a printer, a scanner or a multifunctional device, and more particularly, to a method and apparatus controlling a light power of a laser diode (LD) which is an element for a laser scanning unit (LSU) in a printing device.
2. Description of the Related Art
A conventional laser scanning unit (LSU) includes optical elements, such as a laser diode (LD) emitting laser light, an auto power controller (APC) maintaining a light power of the laser diode (LD) at a constant level, a collimator lens making the emitted laser light as parallel light or convergent light with respect to an optical axis, a polygon mirror scanning laser light that has passed the collimator lens at uniform speed in a horizontal direction, a cylinder lens forming an image of the laser light on the surface of the polygon mirror as a linear shape in the horizontal direction, an F-theta lens polarizing light having a predetermined refractive index with respect to the optical axis and reflected on the polygon mirror at uniform speed in a scanning direction, correcting aberration, and focusing on a scanned surface, an image-formation reflective mirror reflecting the laser light that has passed the F-theta lens and forming an image on the surface of a photosensitive drum of a printing device, a synchronous signal detection light sensor receiving the laser light and making horizontal synchronization, and a synchronous signal detection reflective mirror reflecting the laser light on the synchronous signal detection reflective mirror. The optical elements are installed in a housing and sealed. This prevents the optical elements from being contaminated from a foreign substance, such as dust or dispersion toner. In particular, the light power controller maintaining the light power of the LD at the constant level is disposed on a circuit board. The light power controller includes a laser diode driver, a field effect transistor (FET), a laser diode (LD), a light receiving unit, a variable resistor, and a threshold voltage generator.
The operation of the light power controller will now be described. If an ON signal for emitting light from the LD is input into the light power controller, the FET is conductive by the laser diode driver. If the FET is conductive, a current is applied to the LD, the LD emits light, and the light receiving unit receives light and generates a monitor current. The monitor current and the variable resistor detect a voltage, and the detected voltage is input into the threshold voltage generator. If the input voltage is higher than a threshold voltage, the threshold voltage generator generates a high H signal. If the input voltage is lower than the threshold voltage, the threshold generator generates a low L signal. The FET is turned on or off in response to the high H signal or the low L signal so that light of the LD is controlled. The quantity of light of the LD increases or decreases a value of the variable resistor so that the amount of current supplied to the LD increases or decreases. As such, a desired quantity of light can be fixed. In the conventional art, the light power controller is disposed inside the LSU and controlled by a main controller disposed outside the LSU.
However, the light power controller in a conventional analog manner adjusts the value of the variable resistor measuring the monitor current to output a desired quantity of light, thereby fixing the quantity of light emitted from the LD. Thus, even when the intensity of light should vary according to the speed of a printer, the quantity of light cannot be controlled freely. In addition, since the quantity of light is adjusted using the threshold voltage, the range of adjustment is limited. Since the light power controller is disposed inside the LSU, malfunction may occur in the light power controller due to heat caused by light emitted from the LD or vibration. In particular, due to heat, resistance of the fixed variable resistor may vary. In addition, the variable resistor should be fixed according to the quantity of light when manufacturing.